Much current “high technology” involves the use of precision systems that perform actions or processes on workpieces. The scope of possible actions is wide, including but not limited to cutting, machining, polishing, roughening, forming, applying finishes or coatings, surface treating, patterning, treating, chemically modifying, assembling, inspecting, measuring, etc. In many instances, these actions require very accurate and precise positioning of workpieces, process components, and the like. Also, in many instances the actions are performed under environmental conditions that are or should be controlled to minimize their potentially adverse effects on the process(es) being performed by the system. For example, if a workpiece is excessively heated, particularly while being “worked” on the precision system, the workpiece may undergo excessive thermal deformation, which can decrease the accuracy and precision of the work being performed on the workpiece.
“Workpiece” is not limited to a thing to which the action is being performed by the precision system. Some precision systems use several types of workpieces, such as a first workpiece used to define the work being done by the system to a second workpiece. Examples of multiple types of workpieces are used in various microlithography systems that are important tools used for manufacturing “micro-devices” including ultraminiature electronic circuits, displays, memory chips, processor chips, etc. Most conventional microlithography systems use at least two workpieces: a first workpiece that defines a micro-pattern, and a second workpiece onto which the micro-pattern is reproduced or “transferred” from the first workpiece. The microlithography system basically comprises various holding, positioning, illuminating, and imaging subsystems that collective achieve this pattern reproduction on a series of substrates, often semiconductor “wafers,” in a highly automated and controlled manner. As commercial demands for increasingly miniaturized micro-devices continue, the technical demands placed on precision systems (such as microlithography systems) used for making the micro-devices have become progressively more stringent, requiring more control and more attention being paid to parameters (such as heat-loads) that previously could be ignored.
As microlithography systems have evolved for making progressively miniaturized micro-devices, increasingly smaller wavelengths of electromagnetic radiation have been utilized. In many instances, changing to a smaller wavelength required far-reaching changes in system configuration. For example, most earlier microlithography systems used ultraviolet (UV) light for making pattern exposures. The wavelengths of UV light allowed use of refractive optical systems for illumination and/or imaging. To produce a microlithography system capable of forming smaller pattern features on a wafer than achievable using a UV microlithography system, a gigantic effort is currently underway to develop a practical microlithography system utilizing extreme ultraviolet (EUV) light. Such lithography systems are abbreviated “EUVL” systems. EUVL systems can form active circuit elements having width dimensions of approximately 35 nanometers or less. In EUVL the exposure beam has a wavelength of approximately 13.5 nm that typically is produced by a plasma source such as electric-discharge-pinch plasma or laser-produced plasma. In the latter, a pulsed laser beam heats units of xenon gas or other suitable target material to a very high temperature sufficient to create the plasma. The plasma produces intense light in a range of wavelengths including EUV wavelengths. The light is collected by a mirror and reflected through a filter to remove unwanted (“out-of-band”) wavelengths. The filtered light is formed into an illumination beam by passage through an illumination-optical system. The illumination-optical system shapes and conditions the illumination beam (including making the beam substantially uniform in intensity across its transverse dimension) for illuminating a pattern-defining master called a “reticle” or “mask.” Since there are no known materials useful for making EUV lenses, the illumination-optical system comprises only reflective optical elements.
In EUVL systems the reticle is a planar object that is held by a reticle chuck. The reticle chuck is mounted on a reticle stage that controllably moves the reticle chuck (and hence the reticle) in one or more desired directions of motion. For example, the reticle stage is configured to move the reticle chuck in the X-, Y-, and Z-directions, wherein the plane of the reticle extends in the X- and Y-directions, and the Z-direction is normal to the reticle plane. The reticle stage may also be configured to tilt the reticle in one or more of the θX, θY, and θZ directions.
For use with an EUV illumination beam, the planar reticle is reflective rather than transmissive. (In other lithography systems using longer UV exposure wavelengths, the reticle is usually transmissive.) The illumination beam incident on the reticle reflects from the reticle while becoming “patterned” according to the arrangement of features on the reticle, wherein the arrangement of features on the reticle corresponds to the desired pattern to be exposed onto a lithographic substrate. An example of a “substrate” is a semiconductor wafer coated with an exposure-sensitive material called a “resist.” The EUV-patterned beam reflected from the reticle passes through an imaging-optical system, also entirely reflective, to the resist-coated substrate. The substrate is mounted on a “wafer chuck” affixed to a wafer stage that controllably moves the wafer chuck (and hence the substrate) in one or more desired directions of motion. At least some wafer-stage motion is coordinated with corresponding reticle motion. The wafer stage is configured to move the wafer chuck in the X-, Y-, and Z-directions, wherein the plane of the wafer extends in the X- and Y-directions, and the Z-direction is normal to the wafer plane. The wafer stage may also be configured to tilt the substrate in one or more of the θX, θY, and θZ directions. These motions not only ensure that pattern exposures occur at desired locations on the substrate surface, but also may be performed to ensure good focus of the images as formed on the substrate surface. The plane of the substrate surface (extending in the X- and Y-directions) is usually optically conjugate to the plane of the reticle surface from which the illumination beam reflects.
A mirror arranged to provide glancing reflection of incident EUV light reflects almost all the incident light. However, glancing-incidence mirrors are impractical for most of the mirrors in the illumination-optical system. As a result, most of the mirrors are arranged and configured to reflect light at smaller angles of incidence. In order for a mirror to exhibit acceptable reflectivity to EUV light at less than a glancing angle of incidence, it must have a multilayer coating on the intended reflective surface. Even with a multilayer-coated mirror, the greatest achievable reflectivity of EUV light is approximately 70%, wherein the remaining approximately 30% of the energy of EUV light incident on the mirrors is absorbed as heat. To avoid or minimize thermal distortion, the mirror is at least passively cooled and may be actively cooled. For active cooling, see, e.g., U.S. Pat. No. 7,591,561 to Phillips et al., incorporated herein by reference. An example of “passive” cooling is simple conduction of the heat from the mirror to a mass serving as a heat-sink. An example of active cooling is circulation of a temperature-regulated fluid through conduits in the body of the mirror.
Since the reticle is located just downstream of the illumination-optical system, the reticle is vulnerable to substantial heating. Reticle heating is exacerbated by the fact that the reticle also includes a surficial multilayer film providing the reticle with a maximal achievable reflectivity of approximately 70%, resulting in approximately 30% of incident EUV light being absorbed by the reticle as heat.
Since the reticle defines the profile of the pattern to be formed on the wafer as well as placement information for the elements of the pattern as formed on the wafer, parameters such as flatness (planarity) of the reticle are critical. Other important reasons for reticle flatness include: (a) the illumination-optical system is typically not telecentric, and (b) the imaging-optical system typically has a very small depth of focus.
Heat absorbed by the reticle (especially uneven heat absorption) can cause reticle distortion, which is the antithesis of reticle flatness. The distortion usually results in at least a portion of the reticle surface becoming displaced out of the plane conjugate to the wafer plane, which causes an image-fidelity error, an image-placement error, and/or an image-registration error. In addition, uneven heat absorption by the reticle can cause thermal distortion of the reticle within the plane conjugate to the wafer plane. Thus, temperature gradients both normal to the reticle plane and within the reticle plane can create thermal distortions that impair image quality.
Reticle-flatness specifications as promulgated by an industrial committee (SEMI) are very tight for EUVL. Example recent specifications are:
Feature half-pitchReticle flatness(nm)(nm)455032322223Semi P37-1101, Specification for Extreme Ultraviolet Lithography Mask Substrates (2001). These specifications are particularly directed to the patterned area of the reticle as the reticle is being held by the reticle chuck. The patterned area is the important part of the reticle for imaging purposes and is surrounded by a peripheral zone in which alignment marks, for example, are located. Semi P40-1103, Specification for Mounting Requirements and Alignment Reference Locations for Extreme Ultraviolet Lithography Masks (2003). For increasingly better image resolution these specifications are progressively becoming more stringent. Meanwhile, as EUV sources are developed that produce progressively greater output power to satisfy demands for ever-increasing throughput, the thermal burdens to which EUVL reticles are vulnerable continue to increase.